llvm dev - Nov 2016 - GlobalValue::AvailableExternallyLinkage

Hi,

could anybody explain what GlobalValue::AvailableExternallyLinkage means?

I implemented an instrumentation pass that creates a clone of a function.
For some programs I noticed that also function such as “atoi”, “atof”,
“__strspn_c2”, etc. they get cloned even if I am not implementing them in the
module.
I would like to avoid cloning those functions, so I noticed that they have
GlobalValue::AvailableExternallyLinkage linkage.
Checking if they have GlobalValue::AvailableExternallyLinkage would it be enough
to ignore them?

Thanks.
Best,
Simone

> On Nov 19, 2016, at 12:00 PM, Simone Atzeni via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi,
> 
> could anybody explain what GlobalValue::AvailableExternallyLinkage means?
It means that the function will not be codegen in this module, it will be
available at link time from another object.
The IR is available for the purpose of inlining mainly.
> 
> I implemented an instrumentation pass that creates a clone of a function.
> For some programs I noticed that also function such as “atoi”, “atof”,
“__strspn_c2”, etc. they get cloned even if I am not implementing them in the
module.
> I would like to avoid cloning those functions, so I noticed that they have
GlobalValue::AvailableExternallyLinkage linkage.
> Checking if they have GlobalValue::AvailableExternallyLinkage would it be
enough to ignore them?
I don’t really understand what you’re doing or why, so I can’t answer this
question.

— 
Mehdi

Thanks Mehdi.

My pass clones the functions within a module in order to have the original
function and an exact copy of the same function but with a different name, i.e.
sum() and sum_parallel().
After my pass I will run ThreadSanitizer instrumentation pass only on the new
copy of the functions, i.e. only the “_parallel” functions will be instrumented
by tsan.

In some programs that I am compiling, the functions such as atoi and atof get
cloned but I want to avoid this, and I noticed that only those functions have
GlobalValue::AvailableExternallyLinkage, so I was wondering if checking the 
linkage is enough to avoid those library functions or there could be situations
of functions with that linkage but that have the body implemented in the same
module.

Does it make sense now?

Thanks.
Simone
> On Nov 19, 2016, at 13:32, Mehdi Amini <mehdi.amini at apple.com>
wrote:
> 
> 
>> On Nov 19, 2016, at 12:00 PM, Simone Atzeni via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
>> 
>> Hi,
>> 
>> could anybody explain what GlobalValue::AvailableExternallyLinkage
means?
> 
> It means that the function will not be codegen in this module, it will be
available at link time from another object.
> The IR is available for the purpose of inlining mainly.
> 
>> 
>> I implemented an instrumentation pass that creates a clone of a
function.
>> For some programs I noticed that also function such as “atoi”, “atof”,
“__strspn_c2”, etc. they get cloned even if I am not implementing them in the
module.
>> I would like to avoid cloning those functions, so I noticed that they
have GlobalValue::AvailableExternallyLinkage linkage.
>> Checking if they have GlobalValue::AvailableExternallyLinkage would it
be enough to ignore them?
> 
> I don’t really understand what you’re doing or why, so I can’t answer this
question.
> 
> — 
> Mehdi
>